1. Field of the invention
This invention relates to a semiconductor laser device. More particularly, it relates to a novel structure of semiconductor laser devices which can be used for a laser light source in such fields as optical information processors (e.g., video discs, audio-discs, laser printers, etc.) and/or optical communication systems.
2. Description of the prior art:
Semiconductor laser devices to be used as a light source for optical information processors must have the characteristics that light, with as short a wavelength as possible, can be lased, laser oscillation in a stable transverse mode can be achieved, the threshold current is at a low level, device operation can be carried out for long periods of time, etc. As semiconductor lasers having such characteristics, V-channeled substrate inner stripe (VSIS) lasers are known, an example of which is shown in FIG. 2. On a p-GaAs substrate 1, an n-GaAs current blocking layer 2, a p-GaAlAs cladding layer 3, a GaAlAs active layer 4, an n-GaAlAs cladding layer 5, and an n-GaAs cap layer 6 are successively formed. An n-sided electrode 7 and a p-sided electrode 8 are formed on the upper face of the cap layer 6 and the back face of the substrate 1, respectively. In this semiconductor laser device, current injected thereinto flows through the striped region alone within the V-channel 9 and light outside of the V-channel 9 is absorbed by the current blocking layer 2, so that oscillation in a stable transverse mode at a low threshold current can be easily attained. However, when a laser light-emitting apparatus which can oscillate at a short wavelength of 740 nm or less is produced using the above-mentioned VSIS laser device having such excellent characteristics, there are problems in that distortion (i.e., compressive stress) arises in the active layer 4 and the generation of heat arises in the oscillation region, which significantly shorten the operation life thereof.
The lattice constant of the thick GaAs substrate 1 supporting the epitaxial growth layers thereon completely matches that of each of the GaAlAs growth layers at a growth temperature in the range of about 800.degree. C. to about 900.degree. C., but the thermal expansion coefficient of each is different, so that a difference in the lattice constant therebetween arises at room temperatures, resulting in a compressive stress in the active layer 4. For example, when the active layer is composed of a crystal of Ga.sub.0.7 Al.sub.0.3 As, it receives compressive stress of as high as about 3.times.10.sup.8 dyn/cm.sup.2.
Moreover, the oscillation threshold current Ith is rapidly increased at a wavelength of 740 nm or less, and the Ith at 700 nm or less becomes twice as high as that at 740 nm or more.
In fact, about one half of the Ith of a VSIS semiconductor laser leaks from the optical waveguide and becomes ineffective current which does not contribute to laser oscillation. As mentioned above, Ith is increased at a short wavelength, which causes a considerable generation of heat in the oscillation area. If such a distortion in the active layer and such heat generation in the oscillation area can be reduced, the life of the device will be noticeably improved.